To the Editor.—The diagnostic model presented by Johnson et al1 demonstrates the potential usefulness of receiver operating characteristic (ROC) curves in clinical decision making and provides a method to estimate the probability of Chlamydia trachomatis infection more precisely for the individual patient. Such probability estimates are necessary to determine the predictive value of nonculture test results accurately and thereby provide appropriate recommendations for treatment and patient follow-up. However, where universal screening is not feasible, the clinician is likely to be less interested in probability than in the immediate dichotomous decision of whether to test for infection.In contrast to selective screening criteria with simple dichotomous outcomes such as those used in family planning clinics in Wisconsin,2,3 the model presented by Johnson et al requires the clinician to calculate a clinical score for each patient. This raises questions regarding how practical the scoring system may be for routine